Raman Spectroscopic Fingerprints of Atomically Precise Ligand Protected Noble Metal Clusters: Au₃₈ (PET)₂₄ and Au_38-x...

Distinct Raman spectroscopic signatures of the metal core of atomically precise, ligand-protected noble metal nanoclusters are reported using Au₃₈ (PET)₂₄ and Au_38-x Ag_x (PET)₂₄ (PET = 2-phenylethanethiolate, -SC₂ H₄ C₆ H₅ ) as model systems. The fingerprint Raman features (occurring <200 cm^-1 ) of these clusters arise due to the vibrations involving metal atoms of their Au₂₃ or Au_23-x Ag_x cores. A distinct core breathing vibrational mode of the Au₂₃ core has been observed at 90 cm^-1 . Whereas the breathing mode shifts to higher frequencies with increasing Ag content of the cluster, the vibrational signatures due to the outer metal-ligand staple motifs (between 200 and 500 cm^-1 ) do not shift significantly. DFT calculations furthermore reveal weak Raman bands at higher frequencies compared to the breathing mode, which are associated mostly with the rattling of two central gold atoms of the bi-icosahedral Au₂₃ core. These vibrations are also observed in the experimental spectrum. The study indicates that low-frequency Raman spectra are a characteristic fingerprint of atomically precise clusters, just as electronic absorption spectroscopy, in contrast to the spectrum associated with the ligand shell, which is observed at higher frequencies.